The present invention relates generally to aircraft voice communication radios such as are normally employed in two-way communication with air traffic control facilities. More particularly, the invention relates to means for preventing interference of the type which arises when two or more radios are transmitting on the same channel frequency.
Two-way voice communications between aircraft and ground-based air traffic control facilities are generally carried out by means of VHF communications transceivers, customarily termed "COM" radios. Typically, COM radios employ amplitude modulation (AM) and operate in a frequency band from 118.0 MHz to 135.95 MHz on any of a plurality of selectable discrete channel frequencies, e.g., on any one of 720 defined channels, with 0.025 MHz channel spacing.
At each radio location, whether within an aircraft or at an air traffic control facility, there is typically a manually-operable push-to-talk (PTT) switch connected through a suitable microphone switch circuit for activating the transmitter circuitry and de-activating the receiver circuitry. Thus, the normal or stand-by mode is the receiver mode. In the case of a hand-held microphone, the PTT switch is normally included within a microphone housing including both a microphone element and the PTT switch. A microphone cord includes both audio and PTT control conductors, and teminates in a suitable microphone plug. In other cases, a so-called boom microphone element is attached to a headset assembly, and the microphone switch is located elsewhere, for example, on the aircraft control yoke. In some cases, the microphone switch is even voice activated. In any event, the transmitter circuitry can be activated any time at the discretion of the radio operator, such as the aircraft pilot.
With such a channelized two-way voice communication system, it will be appreciated that only one station at a time can be transmitting on any given channel frequency (within the distance range of the equipment). The consequence of two transmissions at once in many cases is that neither transmission is intelligible at the receiving station. In other words, the particular radio frequency channel is temporarily rendered useless. This is particularly so since amplitude modulation (AM) is employed in the type of aircraft communications radio here concerned, rather than frequency modulation (FM). FM communications systems exhibit a "capture effect" whereby the strongest signal presented to a receiver is heard clearly, to the complete exclusion of the other, weaker transmissions. However, in AM communication systems, all transmissions on a particular channel frequency are heard in the receiver causing mutual interference. Moreover, since the actual carrier frequencies of several transmitters nominally on the same channel frequency are rarely in fact identical, but rather can differ by up to several KHz, hetrodynes or beat notes in the form of one or more squeals are normally heard in an AM receiver when more than one transmitter is on the same channel frequency, increasing the effect mutual interference.
There are, in general, two types of situations which lead to the undesirable result of two simultaneous transmissions on a single radio frequency channel. One of these types of situations results from poor radio operating technique, and the other results from equipment malfunction.
More particularly, in order to ensure that a particular channel frequency is not already in use, proper radio operating technique is to always listen before transmitting on the particular channel frequency. However, in practice, proper technique is not always followed. Occasionally pilots transmit on a channel frequency at the same time someone else, either an aircraft or a ground facility, is transmitting on the same channel. This is known as "stepping on" or "blocking" the other transmission.
Even if proper radio technique is being employed, equipment malfunction can cause an inadvertant transmission to occur. More specifically, the microphone switch circuit is susceptible to undesired activation. This undesired activation typically occurs as the result of a short circuit in the microphone cord, the connectors, or the push-to-talk switch itself. Also, there are instances where a microphone is not properly returned to its holder, and instead, is left in a location (e.g., on a seat) where the push-to-talk switch is inadvertantly actuated through contact with another object. Although one of these typical situations is an equipment malfunction and the other is actually a form of improper operation, the term "undesired activation" of the push-to-talk switch circuit is herein employed to refer to these and similar situations. Also, it may be noted that the term "stuck microphone" or "stuck mic" is commonly employed to refer to this condition, and the term is also employed herein.
In many two-way communications systems, the channel blockage which results from two simultaneous transmissions on the same channel may be no more than a minor annoyance. However, in the context of an aircraft communication system, such situations are potentially hazardous. This is particularly so because, in busy air traffic control situations, a controller may be communicating in rapid sequence with a number of aircraft employing brief messages and terse phraseology to maximize the amount of information which can be communicated in a short period of time. At aircraft speeds, situations develop rapidly, and it is important that aircraft communications proceed in a smooth fashion. The potentially hazardous consequence of a missed communication during a critical phase of flight will be well appreciated.
As a partial solution to the problems, our previous U.S. patent application Ser. No. 466,318, filed Feb. 14, 1983, entitled "STEP-OFF DEVICE FOR AIRCRAFT VOICE COMMUNICATION SYSTEM", now U.S. Pat. No. 4,494,244, is directed to systems and devices for preventing the operator of an aircraft radio from beginning a transmission while someone else is transmitting on the same channel. Since this improper technique is commonly referred to as "stepping on" a transmission, we have termed the device which prevents this a "step-off" device (SOD).
Briefly, our previously-disclosed step-off device (SOD) is appropriately connected to the receiving circuitry of the aircraft communication radio to sense when a selected radio frequency channel is in use. When the selected channel is in use, operation of the associated transmitter circuitry is inhibited even though the push-to-talk switch is operated. A particularly advantageous form of step-off device is an external device intended for retrofit connection to an aircraft voice communication type which has ground connection-activated push-to-talk switch circuitry, and the external form of step-off device advantageously derives its operating power from the push-to-talk switch circuit. Thus, no modification is required to either the radio receiver wiring or the aircraft electrical system which otherwise might be required for the purpose of deriving operating power, nor are batteries required which would necessitate frequent replacement to maintain proper operation. Additional details may be had by reference to our U.S. Pat. No. 4,494,244, the entire disclosure of which is hereby expressly incorporated by reference.
Our previous step-off device however does not address the problem of equipment malfunctions of the type summarized above, which lead to undesired activation of the microphone switch circuit. In the case of a "stuck microphone", our previous step-off device at best waits until no other radio is transmitting before allowing the channel to be blocked. However, once transmission begins, our previous step-off device allows such blockage to continue.
In some respects, a "stuck microphone" is far more serious than one transmission "stepping on" or blocking another because a "stuck microphone" is likely to be of much longer duration. The problem of one transmission "stepping on" another is normally a short-term one, and therefore does not result in long-term channel blockage.